Hepatitis is a disease occurring throughout the world. Hepatitis is generally of viral nature, although, if considered a state of chronic inflammation of the liver, there are other known, non-infectious causes. Viral hepatitis is by far the most common form of hepatitis. The U.S. Centers for Disease Control has estimated that at least 1.8% of the U.S. population has serologic evidence of HCV infection, in the majority of cases associated with chronic active infection. HCV is a positive-stranded RNA virus belonging to the Flaviviridae family and has closest relationship to the pestiviruses that include hog cholera virus and bovine viral diarrhea virus.
The HCV genome is a single-stranded, positive-sense RNA of about 9,600 bp coding for a polyprotein of 3009-3030 amino acids, which is cleaved co- and post-translationally by cellular and two viral proteinases into mature viral proteins (core, E1, E2, p7, NS2, NS3, NS4A, NS4B, NS5A, NS5B). The structural proteins, E1 and E2, are believed to be embedded into a viral lipid envelope and form stable heterodimers. The structural core protein is believed to interact with the viral RNA genome to form the nucleocapsid. The nonstructural proteins designated NS2 to NS5 include proteins with enzymatic functions involved in virus replication and protein processing including a polymerase, protease, and helicase. HCV replicates through the production of a complementary negative-strand RNA template.
HCV is a genetically diverse virus. Within a single infected patient, many variant viruses can be identified, leading to the description ‘viral swarm’, or viral quasispecies. Within the global human population, HCV is also genetically diverse, with at least 6 major ‘genotypes’ identified (Genotypes 1-6), and numerous subtypes (i.e., HCV Genotype 1a and 1b). HCV genotypes are defined by genomic phylogenetic analysis, and diagnosed (in a given patient) by HCV RNA sequence-based diagnostic assays.
The main route of infection with HCV is blood exposure. The magnitude of the HCV infection as a health problem is illustrated by the prevalence among high-risk groups. For example, in some surveys, 60% to 90% of hemophiliacs and more than 80% of intravenous drug abusers in western countries had chronic HCV infection. For intravenous drug abusers, the prevalence varies from about 28% to 80% depending on the population studied. The proportion of new HCV infections associated with blood or blood product transfusion has been markedly reduced due to pharmaceutical advances and widespread use of sensitive serologic and RNA detection assays used to screen blood donors, however, a large cohort of aging, chronically infected persons is already established.
One available treatment for HCV infection is pegylated interferon-α (PEG-IFN α1a or PEG-IFN α1b), which is, under current treatment guidelines, administered weekly by subcutaneous injection for 24 to 48 weeks, dependent upon the HCV viral genotype being treated. Although greater than 50% of patients with Genotype 1 HCV infection may be expected to have suppression of HCV viremia at the completion of 48 weeks therapy, a significant proportion of these patients will have viral relapse. Accordingly, a Sustained Virologic Response (SVR, defined as HCV RNA negativity 24 weeks post treatment cessation, and considered tantamount to ‘cure’) is only achieved in 30-40% of Genotype 1 HCV infections treated with PEG-IFN alone. In addition, treatment with PEG-IFN+RBV is not well tolerated, with an adverse event profile that includes flu-like symptoms, thrombocytopenia, anemia, and serious psychiatric side effects. While treatment with the current standard of care is suboptimal, many patients are precluded from ever starting therapy due to comorbidities common in HCV-infected populations, including psychiatric disorders, advanced liver disease, and substance abuse.
Ribavirin is a nucleoside analog antiviral drug. Ribavirin is typically taken orally (by mouth) twice a day. The exact mechanism for ribavirin is unknown. However, it is believed that when ribavirin enters a cell it is phosphorylated; it then acts as an inhibitor of inosine 5′-monophosphate dehydrogenase (IMPDH). IMPDH inhibitors such as ribavirin reduce the intracellular synthesis and storage of guanine, a nucleotide “building block” necessary for DNA and RNA production, thus inhibiting viral replication. IMPDH inhibitors also interfere with the reproduction of rapidly proliferating cells and cells with a high rate of protein turnover. Treatment with ribavirin monotherapy has little effect on HCV RNA levels, but is associated with a decline in serum alanine transferase (ALT). This observation suggests that ribavirin may not be acting as an antiviral agent, but rather as a modulator of immune system function. Ribavirin is only approved for use, for HCV infection, in combination with IFN.
Treatment with the combination of PEG-IFN plus ribavirin improves SVR rates over those observed with PEG-IFN alone, in large part due to reduction in the frequency of viral relapse at the cessation of therapy. Large clinical trial SVR rates for PEG-IFN/ribavirin treated patients with HCV Genotype 1 infection have ranged from 40-55%. At the present time, PEG-IFN/ribavirin therapy is considered the ‘standard-of-care’ treatment for chronic HCV infection. The standard of care is, however, expected to change rapidly in the near future with approval of direct acting antiviral agents which will, initially, be used in combination with PEG-IFN/ribavirin.
Unfortunately, different genotypes of HCV respond differently to PEG-IFN/ribavirin therapy; for example, HCV genotype 1 is more resistant to therapy than types 2 and 3. Additionally, many current treatments for HCV produce unwanted side effects. Thus, there is currently a need for new anti-viral therapies. In particular there is a need for new antiviral therapies that produce fewer unwanted side-effects, that are more effective against a range of HCV genotypes, or that have less complicated dosing schedules, i.e. that require administration of agents fewer times during a day.